Induction of neurally mediated syncope with adenosine

Background-Tilt testing is used to establish the diagnosis of neurally medi ated syncope. However, applicability of the tilt test is limited by test se nsitivity and length of time required to perform the test. We hypothesized that adenosine could facilitate the induction of neurally mediated syncope through its sympathomimetic effects and therefore could be used as an alter native to routine tilt testing. Methods and Results-In protocol I, the yield of adenosine tilt testing (12 mg while upright, followed by 60 degrees tilt for 5 minutes) and a 15-minut e isoproterenol tilt test were compared in 84 patients with a negative 30-m inute drug-free tilt test. In protocol 2, 100 patients underwent an initial adenosine tilt test followed by our routine tilt test (30-minute drug-free tilt followed by a 15-minute isoproterenol tilt). Six additional control p atients underwent microneurography of the peroneal nerve to compare the sym pathomimetic effects during bolus administration of adenosine and continuou s infusion of isoproterenol. In protocol 1, the yields of adenosine (8 of 8 4 10%) and isoproterenol (7 of 84, 8%) tilt testing were comparable (P=NS). In protocol 2, the yields of adenosine (19 of 100, 19%) and routine (22 of 100, 22%) tilt testing were also comparable (P=NS). Although the yield of adenosine tilt testing was comparable in both protocols, patients with a ne gative adenosine tilt test but a positive routine tilt test usually require d isoproterenol to elicit the positive response. Microneurography confirmed discordant sympathetic activation after adenosine and isoproterenol admini stration. Conclusions-Adenosine is effective for the induction of neurally mediated s yncope, with a diagnostic yield comparable to routine tilt testing. However , the discordant results obtained with adenosine and the isoproterenol phas e of routine tilt testing suggest that adenosine and isoproterenol tilt tes ting may have complementary roles in eliciting a positive response. Therefo re, a tilt protocol that uses an initial adenosine Lilt followed, if necess ary, by an isoproterenol tilt would be expected to increase the overall yie ld and reduce the duration of tilt testing.